Attenuator De-Qing Loss Improvement and Phase Balance

ABSTRACT

The de-Qing loss and phase imbalance caused by the inherent capacitance of a switched resistance, such as a MOSFET with a resistor, can be reduced by using a shunting switch across the resistor that is in series with the resistor&#39;s switch. The shunting switch shorts across the resistor when the resistor&#39;s switch is open and in reference mode, thereby significantly reducing the resistance in series with the inherent capacitance of the open resistor&#39;s switch.

CROSS REFERENCE TO RELATED APPLICATIONS—CLAIM OF PRIORITY

This application is a continuation of commonly owned and co-pending U.S.application Ser. No. 15/212,025, filed Jul. 15, 2016, entitled“Attenuator De-Qing Loss Improvement and Phase Balance”, the disclosureof which is incorporated herein by reference in its entirety.

The present application may be related to U.S. patent application Ser.No. 15/212,046, filed on Jul. 15, 2016, entitled “Hybrid Coupler withPhase and Attenuation Control” (Attorney Docket No. PER-178-PAP), nowU.S. Pat. No. 10,211,801 issued Feb. 19, 2019, and assigned to theassignee of the present disclosure, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

Various embodiments described herein relate generally to systems,methods, and apparatus for improving de-Qing loss and phase balance forattenuator circuits.

BACKGROUND

Various devices can be switched between a reference mode and anattenuated mode by the use of a switched parallel resistance. Forexample, a Pi-pad attenuator can include transistors on the legs of thePi-pad to turn on and off each of the legs from an “attenuation mode”(switched on) to a “reference mode” or “floating mode” (switched off).Ideally, with the transistor turned on the input signal only sees theparallel resistance at the node and with the transistor turned off theinput signal only sees an open circuit at the node. However, transistorsin the off state are not perfect open circuits: they have small internalconductance and capacitance. The capacitance, known as parasiticcapacitance, is of particular concern for signals that contain highfrequency components, as the frequency keeps on increasing capacitancestarts to acts like a short circuit rather than an open circuit asdesired. As it becomes more of a short circuit, any internalresistance/conductance is now seen by the RF signal. But mostdetrimental is the attenuating resistor that was intended to be leftfloating is now seen through the signal path and incurs loss to thesystem. The insertion loss caused by the parasitic capacitance in serieswith internal and external resistance (attenuating) increases as thefrequency increases. This insertion loss can be known as “de-Qing” thecircuit, as it lowers the Q (gain) value. Additionally, the parasiticcapacitance with the parallel resistance degrades phase imbalanceincreasingly as the signal frequency increases. For lower frequencies,these losses might be within acceptable ranges, and so can be ignoredfor some designs; however, for high frequency circuits, the de-Qing andphase imbalance can be an issue.

SUMMARY OF THE INVENTION

According to a first aspect of the present disclosure, a circuit isdisclosed, comprising: a resistive element connected to an inputterminal; a primary switching element in series with the resistiveelement; and a shunting switching element placed across the resistiveelement such that it would short current around the resistive element;wherein the shunting switching element is configured to be closed whenthe primary switching element is open, and the shunting switchingelement is configured to be open when the primary switching element isclosed.

According to a second aspect of the present disclosure, a method ofreducing de-Qing loss for a circuit, said circuit comprising a resistiveelement connected to an input signal and a primary switching element inseries with the resistive element and a shunting switching elementacross the resistive element, is disclosed, the method comprising:opening the shunting switching element when the primary switchingelement is closed; and closing the shunting switching element when theprimary switching element is open.

According to a third aspect of the present disclosure, a polyphasefilter circuit comprising: parallel filter elements comprising aresistor and a capacitor; a primary switch in series with the resistorand the capacitor; and a shunting switch in parallel with the resistorand the capacitor, configured to short across the combination of theresistor and the capacitor when the shunting switch is closed.

According to a fourth aspect of the present disclosure, a method offabricating switchable attenuation circuit with deQuing loss reductionis disclosed, the method comprising: providing a resistive elementparallel to a signal input terminal; providing a primary switchingelement in series with the resistive element; and providing a shuntingswitching element placed across the resistive element such that it wouldshort current around the resistive element; configuring the circuit suchthat the shunting switching element is closed when the primary switchingelement is open, and the shunting switching element is open when theprimary switching element is closed.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of exampleembodiments, serve to explain the principles and implementations of thedisclosure.

FIG. 1 shows a prior art switched pi attenuator pad.

FIGS. 2A and 2B show an example of a switched pi attenuator pad withswitch-shunted resistors. FIG. 2A shows an unbalanced pi attenuator pad,and FIG. 2B shows a balanced pi attenuator pad, or O pad.

FIG. 3 shows an example of a single-pole multiple-throw absorptiveswitch with switch-shunted resistors.

FIG. 4 shows an example of a switched tee attenuator pad withswitch-shunted resistors.

FIG. 5 shows an example of a switched L attenuator pad withswitch-shunted resistors.

FIG. 6 shows an example of a switched balanced attenuator withswitch-shunted resistors.

FIG. 7 shows an example of a switched reflection attenuator withswitch-shunted resistors.

FIG. 8 shows an example of a distributed attenuator with switch-shuntedresistors.

FIG. 9 shows an example graph comparing insertion loss between adistributed attenuator with and without switch-shunted resistors.

FIG. 10 shows an example graph comparing phase balancing between adistributed attenuator with and without switch-shunted resistors.

FIG. 11 shows an example graph comparing insertion loss of aconventional attenuator with and without switch-shunted resistors.

FIG. 12 shows an example graph comparing relative attenuation of aconventional attenuator with and without switch-shunted resistors.

FIGS. 13A to 13D show RC circuit equivalents for a pi attenuation padembodiment in attenuation mode and in reference mode, with and withoutthe shunting switch.

FIG. 14 shows an example of a switchable polyphaser filter with aswitch-shunted RC element.

FIG. 15 shows a second example of a switchable polyphaser filter with aswitch-shunted RC component.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of a known pi (π) pad attenuator circuit,commonly used to reduce the level of a signal. A series resistance (110)of the circuit can be affected by switchable parallel resistances (120),which are controlled by switches (130) set in series with thoseresistances (120). These switches can be known as the “primary switches”or “resistors' switches”. Because switches have an inherent capacitance,each leg (140) of the circuit, when that leg's switch (130) is turnedoff (i.e., put in reference mode), has an equivalent circuit (150) of aresistance (152) in series with a capacitor (153). Typically, theequivalent resistance (152) will be nearly equal to the parallelresistance (120) because the inherent resistance of the switch (130) istypically negligible. This has the effect of de-Qing the circuit at highfrequencies—an increasing fraction of the alternating current (“AC”) islost. Also, a phase imbalance between is created from this unintentionalfilter element. A common implementation of a switch in a circuit is asingle field-effect transistor (FET), although other switching meanscould be used. While a single FET is shown or discussed in the examplesprovided, one of ordinary skill would understand that a stack of FETscan be substituted for the single FET, thereby increasing powerhandling, linearity, and other factors without changing the basic natureof the invention.

FIG. 2A shows an example of the pi pad attenuator circuit with switchedresistance (the de-Qing loss reduction system) to alleviate de-Qing ofthe attenuator and reduce phase imbalance at higher frequencies. Whenthe primary switch (230) of a leg is turned off (reference mode),another switch (225) in parallel with the leg resistance (220) is turnedon, creating a short across the resistance (220). This other switch canbe known as the “shunting switch”. The equivalent circuit (250) of theleg (240) is a capacitance (253) from the primary switch (230) in serieswith a resistance (252) that has a short (255) across it. This createsan equivalent RC filter with almost no resistance. This reduces highfrequency loss and phase imbalance when the attenuator is in referencemode. To return to attenuation mode, the primary switch (230) is turnedon and the shunting switch (225) is turned off, making the equivalentcircuit just a resistance to ground/low voltage. This reduces insertionloss in reference mode and acts as a phase balancer for the referenceand attenuating path. It also improves the high frequency attenuation inthe presence of ground inductance. The gate voltages of the shuntingswitches (225) of the two legs can be tied to the same voltage input toshort across both parallel resistors (220) at the same time. The pi padattenuator is an unbalanced attenuator: the same principle of a de-Qingloss reduction can be applied to the balanced form of the pi padattenuator, the O pad attenuator, such as the split O pad attenuatorshown in FIG. 2B. The use of resistor shunting improves both common modeand differential mode attenuation.

In one embodiment, the switches (225, 230) are MOSFET transistors, andthe gate width (w) of shunting switch (225) is less than the w of theprimary switch (230). For example, the w of the shunting switch (225)can be 1/14 the w of the primary switch (230). In one embodiment, theswitches (225, 230) have the same stack size. And this can be chosen toachieve the desired performance at a desired frequency.

The de-Qing loss reduction system can be applied to other attenuationcircuits. Some, but not all, examples are provided herein. The systemcan also be applied to other circuits that switch resistances to achieveattenuation, filtering, or signal absorption and that would experiencede-Qing at high frequencies. The application of the de-Qing lossreduction system works especially well in a CMOS integrated circuit,where the addition of a transistor is simplified.

FIG. 3 shows an example of the de-Qing loss reduction system a SPDT(single pole, double throw) RF (radio frequency) switch. The switchtoggles between connecting a common RF port (300) to a first port (301)and a second port (302), and any power incident on the non-connectedport gets absorbed through a resistor (320-A or 320-B). Theconnected-port resistor (320-B or 320-A) is then shorted out by ashunting switch 325-B or 325-A). For example, if the SPDT switch (330)connects the first port (301) to the common port (300) and connects thesecond port (302) to the resistor (320-B), then the correspondingshunting switch (325-B) will be off, allowing the second port (302) tosee the resistance, and the other shunting switch (325-A) will be on,shorting across the unused resistor (320-A), thereby reducing de-Qingloss caused by the inherent capacitance of the open end of the SPDTswitch acting as a short for high frequency signals, allowing power tobe attenuated through the parallel resistance. The same principle wouldapply for a SPnT (single pole, multiple throw) circuit, or anycombination of poles and throws for an absorptive switch circuit.

FIG. 4 shows an example of the de-Qing loss reduction system a tee (T)pad attenuator circuit, which works on the same principle of shortingacross the parallel resistance. Note that the gate voltage of theprimary switch (430) is opposite that of the shunting switch (425), astheir on and off state would be opposite of each other in operation.

FIG. 5 shows an example of an L pad attenuator, which works on the sameprinciple as the pi pad attenuator, but with only one leg (540).

FIG. 6 shows an example of the de-Qing loss reduction system with abalanced attenuation circuit. The legs (640) can be placed between theresistive elements (671) that run between the quadrature couplers (681,682).

FIG. 7 shows an example of the de-Qing loss reduction system for areflection attenuator. The legs (740) are connected to terminals of thequadrature coupler (780) to provide variable attenuation. It is notedthat de-Qing loss is maximized by having multiple legs connected to thesame terminal.

FIG. 8 shows an example of the de-Qing loss reduction system for adistributed attenuator. The legs (840) are added between coplanarwaveguides (810) which conduct a signal between the end load elements(820), with an inductor (880) added to each leg. Coplanar waveguides(810) are shown as the transmission lines in this example, but othertypes of transmission lines can be used as well, such as microstrip orstripline.

FIG. 9 shows an example graph of insertion loss (IL) versus signalfrequency for the distributed attenuator of FIG. 8. The IL of theattenuator without using the switches for shunting across the resistanceof the leg (the switch is left open in reference mode) is shown as adecreasing signal power as frequency increases (910). In comparison, thesystem utilizing the switches for shunting across the resistance isshown as decreasing at a lower rate (920). As the frequency increases,the difference (930) of IL between the two systems tends to increase, atleast to a point.

FIG. 10 shows an example graph of phase imbalance versus signalfrequency for the distributed attenuator of FIG. 8. The phase shift ofthe attenuator without using the switches for shunting across theresistance (1010) increases as frequency increases. The phase shift ofthe system with the resistances shunted by the de-Qing loss reductionsystem (1020) increases at a markedly lower rate.

FIG. 11 shows an example graph of insertion loss versus frequency for aconventional attenuator with resistance shunting (1120) and withoutresistance shunting (1110). It can be seen that the insertion loss isimproved for the de-Qing loss reduction system. FIG. 12 shows an examplegraph of relative attenuation of the same conventional attenuator asused for FIG. 11. The relative attenuation for the attenuator withoutthe shunting switch across the resistance (1210) increases at a fasterrate than with the shunting switch (1220). As can be seen with ashunting switch configuration a desired 4 dB [+/−0.5 dB] performance isachieved up to 35 GHz, however the case without it only goes up to 25GHz.

FIG. 13A shows an equivalent circuit diagram as the pi pad attenuationcircuit without a de-Qing loss reduction shunting switch in attenuationmode. FIG. 13B shows an equivalent circuit diagram as the pi padattenuation circuit without a de-Qing loss reduction shunting switch inreference mode. FIG. 13C shows an equivalent circuit diagram as the pipad attenuation circuit of FIG. 2A with a de-Qing loss reductionshunting switch in attenuation mode. FIG. 13D shows an equivalentcircuit diagram as the pi pad attenuation circuit with a de-Qing lossreduction shunting switch in reference mode, configured as shown in FIG.2A. The input impedance (“Zin”) (1300-ZA, 1300-ZB, 1300-ZC 1300-ZD) isconsidered for a leg (1340-A, 1340-R, 1345-A, 1345-R) of the circuit ineach case.

For the non-shunted case in attenuation mode, the leg (1340-A) has a Zin(1300-ZA) is a function of the two equivalent resistances, theresistance of the primary switch in a closed state (1330-R) and theattenuating resistor (1320-R), as shown in FIG. 13A. In reference mode,the leg (1340-R) has a Zin (1300-ZB) as a function of the capacitance ofthe primary switch in an open state (1330-C) and the attenuatingresistor (1320-R), as shown in FIG. 13B. Except for very lowfrequencies, a transition between these modes will cause a phase shift

However, for the shunted resistance case, the phase shift caused by thetransition between attenuation mode and reference mode can be greatlyreduced. As shown in FIG. 13C, the Zin (1300-ZC) of the leg (1345-A) isa function of the resistance of the primary switch in a closed state(1330-R), the attenuating resistor (1320-R), and the capacitance of theshunting switch in an open state (1325-C). When transitioned toreference mode, the Zin (1300-ZD) of the leg (1345-R) is a function ofthe capacitance of the primary switch in an open state (1330-C), theattenuating resistor (1320-R), and the resistance of the shunting switchin a closed state (1325-R). With proper selection of shunting switchsize, shunt arm impedance Zin in attenuating mode (1300-ZC) andreference mode (1300-ZD) can be made close to each other over a certainfrequency range. This would reduce the phase shift caused when changingmodes.

FIG. 14 shows an example of a polyphase filter. By proper choice ofresistance and capacitance values for the circuit, the outputs (1402,1403) can become a phase offsetted value of the input (1401) signal.

FIG. 15 shows an example of a tunable polyphase filter where the RCvalues of the filter can be arbitrarily selected through switches,allowing for different phase offsetting. One RC component (internal tothe region 1530) is shunted (1525) while turned off to reduce the deQingloss, and poly phase filtering is done by the other RC component(external to the region 1530). This is just one embodiment of thefilter: the presented idea should be valid to any filtering system whereswitched resistance is used to change the filter characteristic.

Fabrication Technologies and Options

The term “switch” herein includes any technology that has anelectronically (or optically) controllable resistance which can togglebetween an open (very high resistance) state to a closed (very lowresistance) state in a very short period of time, which exhibits acapacitance in the open state. This function can be performed by amechanical switch, a transistor (such as a MOSFET or MESFET), or a smallmechanical switch (such as a microelectromechanical systems (MEMS)switch).

The term “MOSFET” technically refers to metal-oxide-semiconductors;another synonym for MOSFET is “MISFET”, formetal-insulator-semiconductor FET. However, “MOSFET” has become a commonlabel for most types of insulated-gate FETs (“IGFETs”). Despite that, itis well known that the term “metal” in the names MOSFET and MISFET isnow often a misnomer because the previously metal gate material is nowoften a layer of polysilicon (polycrystalline silicon). Similarly, the“oxide” in the name MOSFET can be a misnomer, as different dielectricmaterials are used with the aim of obtaining strong channels withsmaller applied voltages. Accordingly, the term “MOSFET” as used hereinis not to be read as literally limited to metal-oxide-semiconductors,but instead includes IGFETs in general.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of specifications. Unless otherwise noted above, selection ofsuitable component values is a matter of design choice and variousembodiments of the invention may be implemented in any suitable ICtechnology (including but not limited to MOSFET and IGFET structures),or in hybrid or discrete circuit forms. Integrated circuit embodimentsmay be fabricated using any suitable substrates and processes, includingbut not limited to standard bulk silicon, silicon-on-insulator (SOI),silicon-on-sapphire (SOS), GaAs pHEMT, and MESFET technologies. However,the inventive concepts described above are particularly useful with anSOI-based fabrication process (including SOS), and with fabricationprocesses having similar characteristics. Fabrication in CMOS on SOI orSOS enables low power consumption, the ability to withstand high powersignals during operation due to FET stacking, good linearity, and highfrequency operation (in excess of about 10 GHz, and particularly aboveabout 20 GHz). Monolithic IC implementation is particularly useful sinceparasitic capacitances generally can be kept low by careful design.

Voltage levels may be adjusted or voltage and/or logic signal polaritiesreversed depending on a particular specification and/or implementingtechnology (e.g., NMOS, PMOS, or CMOS, and enhancement mode or depletionmode transistor devices). Component voltage, current, and power handlingcapabilities may be adapted as needed, for example, by adjusting devicesizes, serially “stacking” components (particularly FETs) to withstandgreater voltages, and/or using multiple components in parallel to handlegreater currents. Additional circuit components may be added to enhancethe capabilities of the disclosed circuits and/or to provide additionalfunctional without significantly altering the functionality of thedisclosed circuits.

A number of embodiments of the invention have been described. It is tobe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, some of thesteps described above may be order independent, and thus can beperformed in an order different from that described. Further, some ofthe steps described above may be optional. Various activities describedwith respect to the methods identified above can be executed inrepetitive, serial, or parallel fashion. It is to be understood that theforegoing description is intended to illustrate and not to limit thescope of the invention, which is defined by the scope of the followingclaims, and that other embodiments are within the scope of the claims.

1. (canceled)
 2. An attenuator comprising: an attenuator leg comprising:a resistive element connected between an input or output of theattenuator and a ground node of the attenuator, the resistive elementhaving a resistance; a primary switching element in series with theresistive element; and a shunting switching element in parallel with theresistive element; wherein the attenuator leg is configured to beoperated in two primary states of a) the primary switching element openand the shunting switching element closed, or b) the primary switchingelement closed and the shunting switching element open; and wherein theattenuator leg is configured such that a phase shift of the attenuatorbetween the two primary states is less than what a second phase shift ofthe attenuator is between the two states if there is no shuntingswitching element in the at least one attenuator leg.
 3. The attenuatorof claim 2, wherein the phase shift is at least 2.5 degrees less thanthe second phase shift over a portion of a frequency range.
 4. Theattenuator of claim 2, wherein the phase shift is less than half thesecond phase shift over a portion of a frequency range.
 5. Theattenuator of claim 2, wherein an insertion loss of the attenuator overthe phase shift is at least 0.25 dB less than the insertion loss overthe second phase shift.
 6. The attenuator of claim 2, wherein theprimary switching element and the shunting switching element areMOSFETs, each having a gate width.
 7. The attenuator of claim 6, whereinthe gate width of the primary switching element is at least four timeslarger than the gate width of the shunting switching element.
 8. Theattenuator of claim 2, wherein the attenuator is a pi pad attenuator, aT pad attenuator, an L pad attenuator, or an O pad attenuator.